1. Field of the Invention
This invention relates to a processing method for a wafer wherein a laser beam is irradiated upon a wafer such as an optical device wafer to form a modified layer in the inside of the wafer and then external force is applied to the wafer to divide the wafer into individual devices.
2. Description of the Related Art
An optical device wafer wherein a semiconductor layer (epitaxial layer) of gallium nitride (GaN) or the like is formed on a substrate for crystal growth such as a sapphire substrate or a SiC substrate and a plurality of optical devices such as LEDs are partitioned by streets (scheduled division lines) formed in a lattice shape and formed in the semiconductor layer, is comparatively high in Mohs hardness and difficult to divide by means of a cutting blade. Therefore, the optical device wafer is divided into individual optical devices by irradiation of a laser beam, and the divided optical devices are utilized in various electric apparatus such as portable telephone sets, lighting systems, liquid crystal television sets and personal computers.
As a method for dividing an optical device wafer into individual optical devices using a laser beam, first and second processing methods to be described below are known. The first processing method is a method wherein the focal point of a laser beam of a wavelength (for example, 1064 nm) having permeability through a substrate is positioned in the inside of a substrate corresponding to a scheduled division line. Then, the laser beam is irradiated along such scheduled division lines from the rear face side, on which no semiconductor layer is formed, to form a modified layer in the inside of the substrate. Thereafter, external force is applied to divide the optical device wafer into individual optical devices (refer to, for example, Japanese Patent No. 3408805).
The second processing method is a method wherein a laser beam of a wavelength (for example, 355 nm) having absorbability by a substrate is irradiated upon a region corresponding to scheduled division lines from the front face side to form dividing starting grooves which serve as start points of division by abrasion processing. Thereafter, external force is applied to divide the optical device wafer into individual optical devices (refer to, for example, Japanese Patent Laid-open No. Hei 10-305420). Any of the processing methods can divide an optical device wafer with certainty into individual optical devices.
Generally, on the rear face of an optical device such as an LED (Light Emitting Diode), a reflecting film is formed in order to enhance the extraction efficiency of light. This reflecting film is formed on the rear face of a wafer by sputtering, CVD (Chemical Vapor Deposition) or the like in a state of an optical device wafer. Where a reflecting film configured from gold, aluminum or the like is formed on the rear face of an optical device wafer in this manner, there is a problem that a laser beam cannot be irradiated from the rear face side of the optical device wafer. In order to solve this problem, a dividing method for a sapphire wafer wherein, before a reflecting film is formed on the rear face of a wafer, a laser beam is irradiated from the rear face side of the wafer to form a modified layer serving as a start point of division along a scheduled division line in the inside of the substrate is disclosed in Japanese Patent Laid-open No. 2011-243875.